With the increased use of graphite epoxy, particularly in aerospace, there is an increasing demand for a field-applicable, nondestructive method to detect and to measure the severity of service-incurred damage in this material. Ideally, a field applicable system is safe, portable, provides manual or rapid automated scanning, and allows real time interpretation. The method also must provide for single-sided inspection since frequently only one surface is accessible.
Impact is a common source of service-incurred damage in graphite epoxy components and, unfortunately, there is no such thing as "typical" impact damage. Impact damage may consist of varying densities of matrix cracking, delamination (separation of adjacent plies or lamina) and broken fibers. Low levels of impact cause delamination. Higher levels cause more extensive delamination and fiber breakage as well. There may be no visible damage on the impacted surface, yet subsurface damage may be extensive. The distribution of the damage depends on the thickness of the material and the extent to which it is supported as well as on the shape and consistency of the impactor and the force of the impact. In thin, unsupported panels the area of the damage tends to increase towards the back surface while the reverse may be true in thicker materials.
The effect of the damage depends on the type of stress to which the composite is subjected. Compressive strength is affected by delamination while fiber breakage has a greater effect on tensile strength. In both cases the residual strength depends on the amount of damage sustained; not only on the area of the damage measured in planes parallel to the surface but on the through-thickness extent of the damage as well.
The ultrasound pulse-echo A-scan method, known to those of ordinary skill in the materials inspection art, effectively detects delamination in graphite epoxy. Of the available technologies, this ultrasound method most closely meets the requirements for field-application, consequently it is the method that is currently used to inspect graphite epoxy for service incurred damage. Delamination can be detected by a manual scan using a contact transducer. The total area of the damaged region can also be imaged by a conventional C-Scan. The distance between the scanned surface and the delamination can be estimated from a pulse-echo A-Scan. Ultrasound field techniques have two generally recognized drawbacks: They require the use of a couplant material and, perhaps more important, the delamination nearest the scanned surface can mask delaminations that may be deeper in the material. The extent of deeper delamination has profound effects on the residual strength of the material.
In contrast, a properly optimized eddy current system, utilizing techniques described herein, is sensitive to the broken fibers associated with extensive delamination. The sensitivity of pulse-echo ultrasound under many circumstances is independent of the through-thickness extent of the damage. Eddy current is sensitive only to the broken fibers which tend to be associated with delamination extending through several plies. Combining the two methods thus provides an indication of the degree of severity of the damage detected by ultrasound. When the component is to be subjected only to tensile stress, or when only more severe delamination is of concern, Applicant's techniques will be the most efficacious inspection method available.
With the herein described method, eddy current can provide a measure of the thickness of the undamaged material between the surface and the subsurface broken fiber damage. This is analogous to the distance between the scanned surface and the nearest delamination provided by an ultrasonic A-Scan.
Currently available eddy current systems are oriented towards the inspection of surface defects on metal and for the through thickness inspection of thin metal foils or thin-walled tubing. Prior art techniques fail to address thicker metals or weakly conducting composites. By the proper selection of frequency and probe size, the disclosed method provides a capability to inspect these thicker metals and composites.
An example of the state of the art in materials testing is represented by Sugiyama, U.S. Pat. No. 4,739,261 filed April 19th, 1988 in which techniques are disclosed for inspecting ferromagnetic materials. In addition to the limitation of restricting the techniques to ferromagnetic materials, this reference does not address the problems inherent in testing thick materials and composites. In addition, standards in the form of calibration curves must be prepared in advance.
A related application by the same inventor filed Jan. 9, 1989, Ser. No. 294,622, entitled, Eddy Current Method for Measuring Electrical Resistivity and Device for Providing Accurate Phase Detection, discloses a method for measuring resistivity of carbon fiber reinforced composites and other weakly conducting materials.
The ability to measure the defect depth in a material has heretofore required calibration standards. These calibration standards require a test sample of material containing well characterized defects. The test material must be identical with the material to be inspected and the calibration defects must be representative of those expected to be located by the inspection. These standardization requirements severely limit the ability to quantify depth of damage in composites. With the spectrum of materials and defects growing ever larger, a need for an inspection method free of these calibration requirements is needed.
The inability to apply eddy current inspection techniques to thick metal components efficiently is generally recognized in the art. Methods disclosed herein provide this capability to materials thicker than those generally associated with eddy current techniques.
Therefore, it is an object of this invention to teach a method of nondestructive inspection of electrically conductive materials using eddy current techniques.
It is another object of the instant invention to disclose a method for nondestructive inspection that is efficacious when inspecting carbon fiber reinforced composites.
It is yet another object of the present invention to teach a method of nondestructive inspection that can estimate the distance between the scanned surface and a subsurface flaw to include internal broken fiber damage in a carbon fiber composite with a nonmetallic matrix.
It is still another object to disclose a method of nondestructive inspection that can estimate the distance between the scanned surface and subsurface flaws including matrix defects in metallic components.
It is a further object of the methods taught herein to provide the ability to nondestructively inspect metallic components for back surface breaking cracks or defects.
It is another object of the present invention to provide a method of nondestructive inspection of composites that can be performed with an appropriate probe and an assembly of off-the-shelf commercial electronics.
It is yet another object to provide a method of eddy current inspection of composites that can be performed with access restricted to a single surface of the material under test.
It is yet another object to teach a method of inspecting thick carbon fiber reinforced composite sections such as those developed for submersibles and space applications.
It is a further object to teach a method of inspecting carbon fiber reinforced composite sections such as those employed in the fabrication of pressure vessels.
It is yet another object to disclose a method of inpecting carbon fiber reinforced rocket nozzle components.
It is a further object to teach a method of eddy current inspection of materials free of the need for calibration standards.
It is another object to disclose an eddy current inspection method which utilizes the features of a ferrite cup-core probe.
Other objects, advantages and novel features of the invention will appear from a reading of the following detailed description of the invention when considered in conjunction with the accompanying drawings.